The present invention relates to a machine tool for working sheet metal.
As is known, machine tools currently commercially available comprise a rotatable turret supporting a plurality of pins in a peripheral ring, which pins can slide axially under the action of a striker positioned above the turret and controlled mechanically or by means of a hydrodynamic system. By striking on the head of the pin the striker causes a tool supported by this latter and shaped for example as a punch or as a nibbler, to perform a punching or nibbling operation on a portion of sheet metal positioned beneath the turret. Tools utilised in such machines can be of various types. A first type of punch, for example, comprises a central cylindrical portion, an upper cylindrical portion which in use is fixed to the lower end of the tool-carrier pin, and a lower portion the end of which can cut a portion of sheet metal. This lower portion is lodged in a plastics material sleeve the longitudinal dimensions of which are greater than that of the lower portion itself. Beneath the turret the sheet metal rests on a second rotatable turret positioned in a cavity formed in the worktable of the machine. This second turret supports, in a ring around its own periphery, a plurality of dies equal in number to the punches. Such dies are the elements in which are formed respective holes the outline of which is similar to the outline of the lower portion of the punch in such a way that when this latter acts on the sheet metal the portion cut from this latter is carried into the interior of the hole of the die and from which it is then expelled. During punching the plastics sleeve striking the body part of the die is restrained thereby allowing the lower part of the punch to extend out from its longitudinal hole, which punch first cuts the sheet metal and then pushes the cut portion into the cavity in the associated die. A second type of punch tool differs from that just described by the fact that it includes a spring member in place of the plastics sleeve. This latter includes a first fixed plate having a central hole engaged by the central portion of the punch, a second, movable, plate parallel to the first plate and having a central hole housing the lower portion of the punch, and a plurality of springs positioned between the two plates. In the rest position the end of the lower portion of the punch remains within the hole in the movable plate. During operation of the machine the spring member behaves as the plastics spring just described. The nibbling tools operate in the same way as described for punches but perform exclusively the nibbling operation.
Machine tools described above have various disadvantages.
In particular, in such machines the adjustment of the stroke of the tool carrier pin is complex to achieve and at times insufficiently precise. The stroke of the tool carrier pin is a function of the thickness of the sheet metal to be cut or nibbled and the distance, when punches are being used, between the cutting point of the lower portion of the punch and the lower end of the longitudinal hole (housing such lower portion) of the plastics sleeve or of the movable plate, in dependence on the type of punch used. Obviously, adjustment of the stroke of the tool carrier pin is required whenever the thickness of the sheet metal to be cut or nibbled is varied, or when the distance between the cutting point of the punch and the lower end of the plastics sleeve varies. This distance varies when material is removed from the said tip by sharpening it after wear. Naturally the same considerations apply when nibbling tools are to be sharpened. Currently the adjustment of the starting point of the stroke of the pin is effected by inserting a plate of predetermined thickness over the head of the pin. The thickness of such plate is sometimes not determinable a priori and therefore the required correct adjustment is obtained by trial and error, that is to say by trying plates of different thickness and subsequently testing the operation of the tool on the sheet metal. It is apparent that such adjustment requires a long time, which is reflected in production costs.